1. Field of the Invention
The present invention relates to a solid state imaging apparatus and an imaging apparatus, and a method of manufacturing the solid state imaging apparatus, and more particularly to an improved technique for easily enabling a negative potential to be applied to a lower part of a light shielding film, thereby suppressing a smear current.
2. Description of the Related Art
For example, a solid state imaging device to be used in a digital camera, particularly, a conventional solid state imaging device using CCD (Charge Coupled Devices) will be described with reference to FIGS. 11, 12A and 12B. FIG. 11 is a typical view showing a section corresponding to approximately one pixel of the solid state imaging device and FIGS. 12A and 12B are enlarged sectional views showing the vicinity of a device isolating region in FIG. 11, FIG. 12A illustrating a light shielding film of only W and FIG. 12B illustrating a lamination structure in which Ti/TiN films are additionally provided under the light shielding film of W.
A p-type semiconductor layer 5 is formed on an N-type semiconductor substrate 3 of a solid state imaging device 1, and an N-type region portion 7 is provided in a surface part of the p-type semiconductor layer 5. Consequently, a photodiode 7 to be a photoelectric converting portion for carrying out a photoelectric conversion together with the p-type semiconductor layer 5 (the N-type region portion 7 will be hereinafter referred to as the photodiode) is formed. A device isolating region (p region) 9 is provided on an adjacent pixel side of the N-type region portion 7 and an N region (a vertical charge transfer path) 13 is provided on an opposite side through a reading gate portion 11. The N region 13 constitutes an embedded channel of the vertical transfer path in the solid state imaging device 1.
A high concentration impurity surface layer 15 of a reverse conductive type (a p type) is provided in a surface part of the N-type region portion 7. Since the high concentration impurity surface layer 15 is provided, a free electron generated as a dark current is caught by a hole of the high concentration impurity surface Layer 15 so that a dark current component can be prevented from appearing as a scratch in an image. The high concentration impurity surface layer 15 according to the embodiment is provided by a division into a central high concentration portion (a p+ region) 15a on the surface of the N-type region portion 7 and a low concentration portion (a p− region) 15b in a peripheral part thereof. By setting the periphery to be the low concentration portion 15b, it is possible to reduce an electric field in the peripheral part, thereby dropping a voltage at which a charge stored in the photodiode (the N-type region portion) 7 is read onto the embedded channel 13 of the vertical transfer path.
An uppermost surface of the semiconductor substrate 3 in which the photodiode 7 and the embedded channel 13 are formed is covered with an insulating layer 17. A vertical transfer electrode film (for example, a polysilicon film) 19 is provided on the insulating layer 17 just above the embedded channel 13. A light shielding film 23 formed by a metal film is provided on the vertical transfer electrode film 19 through an insulating layer 21. An opening 23a is provided on the light shielding film 23 just above each photodiode 7, and a light is incident into the N-type region portion 7 via the opening 23a. 
The insulating layer 17 is constituted by an insulating layer 17a provided under the vertical transfer electrode film 19 and an insulating layer 17b provided under the light shielding film 23, and forms an ONO structure including oxide films (SiO2) 25a and 25b, nitride films (SiN) 27a and 27b, and oxide films (SiO2) 29a and 29b shown in FIG. 12A. As shown in FIG. 12B, moreover, titanium (Ti) 31 and titanium nitride (TiN) 33 may be provided between the light shielding film 23 and the oxide film 29b. The Ti/TiN films are provided in order to prevent the light shielding film 23 from being peeled.
In the solid state imaging apparatus 1 constituted as described above, it is necessary to suppress the generation of a smear to be a peculiar noise. The smear appears as a longitudinal line when a part in a light receiving surface is exposed to a strong incident light and a generated signal charge overflows from a pixel and enters the charge transfer portion. Also in the case in which a signal charge generated in a deep part of the substrate enters the charge transfer portion, moreover, the smear is caused.
W (tungsten) or W/TiN/Ti to be the light shielding film 23 of the CCD is usually connected to GND. From the past investigation, it has been found that a carrier diffused over a surface P layer mainly causes a smear in the recent CCD which is sufficiently shielded and a change is made depending on a difference in a potential of the surface Player. It is desirable that the potential applied to a surface should be reduced as greatly as possible when the smear is to be improved. As a proposal, a negative potential is applied to the light shielding film which is currently connected to the GND.
For example, a solid state imaging apparatus disclosed in JP-A-7-153932 comprises potential applying means for applying a predetermined potential to a light shielding film. By the application of the predetermined potential to the light shielding film coming in contact with a substrate, a potential of a sensor interface is set to be equal to or lower than a quasi Fermi level in a sensor surface region, thereby suppressing the generation of a false signal. By an application of a lower potential than a surface potential of a sensor portion in a receipt of a light through the sensor portion, moreover, a minority carrier generated by a photoelectric conversion is discharged to the light shielding film to reduce a recombination probability of an electron and a hole.
However, the light shielding film is usually connected to the GND. In order to apply a negative potential other than the GND, therefore, it is necessary to take a terminal out, and to change the applied potential into a pulse and to apply the pulse. For this reason, there is a disadvantage that manufacturing steps are considerably increased and a productivity is thus reduced, resulting in an increase in a manufacturing cost. In case of a solid state imaging device to be used in an inexpensive household product, particularly, it is necessary to reduce a consumed power and to decrease the number of power supplies. Under the actual circumstances, therefore, a ground potential is to be applied as an intermediate potential to a gate electrode. Under present conditions, consequently, a negative bias cannot be applied as the intermediate potential and a potential barrier cannot be formed in an entrance of a vertical charge transfer portion in the conventional solid state imaging device.